1. Field of the Invention
The present invention generally relates to an electrophotographic image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunction machine having at least two of these capabilities, a method of discharging developer from a development device, and a computer program product thereof.
2. Description of the Background Art
Two-component developer consisting essentially of toner and carrier is widely used in electrophotographic image forming apparatuses. Two-component developer is degraded over time while being used in image development and typically comes to the end of its useful life after being used in image development of several hundreds of thousands to several millions of sheets of recording media. If degraded developer is not replaced with fresh developer but is used in subsequent image formation, toner particles therein are not charged sufficiently and can be scattered in the backgrounds of output images or around the interior of the apparatus. Therefore, degraded developer must be removed from the development device, after which the development device is filled with fresh developer to prevent such image failure or contamination of the image forming apparatus.
Several approaches have been tried to remove developer from the development device efficiently and to reduce the time required to do so.
For example, to remove developer from the development device, JP-H06-230668-A proposes rotating developer conveyance members such as screws in a direction reverse to the direction in which the development device is usually driven. More specifically, JP-H06-230668-A discloses a method of discharging developer from a developer container in a development device including two screws for transporting developer, and a development roller serving as a developer carrier.
In this method, when developer is discharged from the development device through a developer discharge port formed therein, the screws are rotated in the reverse direction (hereinafter “reverse rotation”) to their normal rotational direction in image formation so as to discharge developer accumulated on the bottom of the developer container. Subsequently, developer adhering to the development roller is scraped off by a scraper and falls to the bottom of the developer container with the development roller rotated in the normal direction. Then, the screws are rotated in the reverse direction to discharge the developer thus scraped off outside the development device.
It is to be noted that, differently from such development devices, development devices may include a developer container divided into, for example, three compartments (i.e., supply, collection, and agitation compartments) and three corresponding screws, (i.e., supply, collection, and agitation screws), each having a different capability, in addition to the development roller. The collection compartment receives developer from the development roller, and the collection screw provided therein transports the developer collected from the development roller to the agitation compartment. Because toner included in the developer is consumed in image development and accordingly the concentration of toner in the collected developer is reduced, toner is supplied to the collected developer. The agitation screw provided in the agitation compartment mixes the supplied toner and the collected developer so that the mixture has a uniform toner concentration similar to that of developer before image development. The supply screw provided in the supply compartment then supplies the developer having a uniform toner concentration to the development roller.
If the method of the above-described approach is applied to such development devices having three screws and the rotational direction of the screws is reversed abruptly, the developer collected from the development roller and the developer transported reversely by the supply screw are present in the supply compartment at the same time. In other words, a large amount of developer is present in the supply compartment, and with this sudden influx of developer, the supply screw may be damaged by the confluence of the collected developer and the developer transported by the supply screw. Moreover, even if the supply screw is not damaged, such a large amount of developer can impart a mechanical impact to toner in the developer, thus causing toner therein to coagulate. If remaining in the development device after fresh developer is supplied thereto, the coagulated toner is mixed with the fresh toner, thus producing cores of toner. The number of cores of toner can be amplified and degrade image quality. Thus, it is not possible to apply the above-described known method as is to development devices including three developer conveyance members.
Additionally, it is possible that an intermediate transfer belt might be damaged. Because toner contributes to lubrication of photoconductor drums (image carriers), when toner is not supplied to the photoconductor drum after developer is discharged from the development device, the photoconductor drum and the intermediate transfer belt can be left in direct contact with each other for a long time. As a result, not only the photoconductor drum but also the intermediate transfer belt can be damaged.
By contrast, it is possible that it can take long to discharge developer by rotating the three screws only in the normal direction in development in which developer is supplied upward from the supply compartment to the development roller. More specifically, in such development devices, developer should be accumulated in a downstream end portion of the supply compartment by the screw rotating in the normal direction (hereinafter “normal rotation”) to be pushed up from the supply compartment to the development roller against the force of gravity. Therefore, efficiency in removal of developer will decrease after the amount of developer remaining in the development device is reduced substantially. Moreover, when the amount of developer in the development device is substantially reduced, there can be toner particles separated from developer in the development device, and it is difficult to discharge such separated toner particles outside the device.
Therefore, the inventor of the present invention recognizes that there is a need to reduce time required for removal of developer from the development device and downtime of the image forming apparatus without damaging the apparatus, which known approaches fail to do.